Currently, various kinds of portable electrical apparatuses such as notebook computers, personal digital assistants (PDA) and mobile phones need a miniature power generator. Conventional rechargeable batteries are limited by their weight and charging convenience for use, and therefore several power generators have been developed to replace them. Among these power generators, fuel cells fabricated by the MEMS method have been well and rapidly developed because they have the advantages of fast response, high heat dissipation and miniaturized size.
Fuel cells utilize an electrochemical reaction to decompose a fuel into hydrogen ions and electrons together so that a current occurs. Hydrogen gas, methanol, natural gas or gasoline can be employed as the fuel. FIG. 1 is a schematic diagram showing the structure of a direct methanol fuel cell (DMFC) 10. Methanol dilute solution is fed into the channels of an anode 11, and diffuses into a porous gas diffusion layer (GDL) 121. When methanol reaches the catalyst layer 131, the following reaction occurs.
Oxidation Reaction : CH3OH+H2O→CO2+6H−+6e−
In this regard, six electrons collected at the gas diffusion layer 121 move along an external circuit to drive a load 80 and reach another gas diffusion layer 122 of the cathode 15. Simultaneously, carbon dioxide is produced after the oxidation reaction, and needs to be vented into the surroundings. A porous membrane 14 between the gas diffusion layers 121 and 122 separates the anode 11 from the cathode 15 and allows the protons to move across it to reach a catalyst layer 132 so that the following reduction reaction occurs therein.
Reduction Reaction : 3/2O2+6H−+6e−→3H2O
Oxygen necessary for the reduction reaction is blown by an air blower into the gas diffusion layer 122 through the channels of the cathode 15, and meanwhile, water produced at the cathode 15 are blown off toward the external surroundings. The total reaction of the oxidation and the reduction action is illustrated with the following formula.
Total Reaction : CH3OH+3/2O2→CO2+2H2O
However, a micro pump is necessary for the fuel cell 10 to feed the methanol solution and drain surplus liquid. In this regard, the micro pump needs to be supplied with power so that the performance of the fuel cell 10 is lowered. Moreover, a power-consumption blower is also necessary for the fuel cell 10 to vent the water produced after the reduction reaction because the oxygen cannot be fed into the fuel cell if the water exists in the channels of the cathode 15. The conventional fuel cell 10 has an unresolved problem in which the methanol passes through the porous membrane 14 by diffusion and reacts with the oxygen at the cathode 15. Therefore, the operation voltage of the fuel cell 10 is degraded.
In conclusion, a highly efficient fuel cell is in urgent demand for the portable apparatuses. Such a fuel cell can make the best use of a fuel and efficiently transform it into electrical power.